Chloride conductive and cotransport mechanisms in cultures of canine tracheal epithelial cells measured by an entrapped fluorescent indicator

Chloride flux in phagocytes

Phagocytes, such as neutrophils and macrophages, engulf microbes into phagosomes and launch chemical attacks to kill and degrade them. Such a critical innate immune function necessitates ion participation. Chloride, the most abundant anion in the human body, is an indispensable constituent of the myeloperoxidase (MPO)-H2 O2 -halide system that produces the potent microbicide hypochlorous acid (HOCl). It also serves as a balancing ion to set membrane potentials, optimize cytosolic and phagosomal pH, and regulate phagosomal enzymatic activities. Deficient supply of this anion to or defective attainment of this anion by phagocytes is linked to innate immune defects. However, how phagocytes acquire chloride from their residing environment especially when they are deployed to epithelium-lined lumens, and how chloride is intracellularly transported to phagosomes remain largely unknown. This review article will provide an overview of chloride protein carriers, potential mechanisms for phagocytic chloride preservation and acquisition, intracellular chloride supply to phagosomes for oxidant production, and methods to measure chloride levels in phagocytes and their phagosomes.

The effect of NO-donors on chloride efflux, intracellular Ca(2+) concentration and mRNA expression of CFTR and ENaC in cystic fibrosis airway epithelial cells

Since previous studies showed that the endogenous bronchodilator, S-nitrosglutathione (GSNO), caused a marked increase in CFTR-mediated chloride (Cl(-)) efflux and improved the trafficking of CFTR to the plasma membrane, and that also the nitric oxide (NO)-donor GEA3162 had a similar, but smaller, effect on Cl(-) efflux, it was investigated whether the NO-donor properties of GSNO were relevant for its effect on Cl(-) efflux from airway epithelial cells. Hence, the effect of a number of other NO-donors, sodium nitroprusside (SNP), S-nitroso-N-acetyl-DL-penicillamine (SNAP), diethylenetriamine/nitric oxide adduct (DETA-NO), and diethylenetriamine/nitric oxide adduct (DEA-NONOate) on Cl(-) efflux from CFBE (∆F508/∆F508-CFTR) airway epithelial cells was tested. Cl(-) efflux was determined using the fluorescent N-(ethoxycarbonylmethyl)-6-methoxyquinoliniu bromide (MQAE)-technique. Possible changes in the intracellular Ca(2+) concentration were tested by the fluorescent fluo-4 method in a confocal microscope system. Like previously with GSNO, after 4 h incubation with the NO-donor, an increased Cl(-) efflux was found (in the order SNAP>DETA-NO>SNP). The effect of DEA-NONOate on Cl(-) efflux was not significant, and the compound may have (unspecific) deleterious effects on the cells. Again, as with GSNO, after a short (5 min) incubation, SNP had no significant effect on Cl(-) efflux. None of the NO-donors that had a significant effect on Cl(-) efflux caused significant changes in the intracellular Ca(2+) concentration. After 4 h preincubation, SNP caused a significant increase in the mRNA expression of CFTR. SNAP and DEA-NONOate decreased the mRNA expression of all ENaC subunits significantly. DETA-NO caused a significant decrease only in α-ENaC expression. After a short preincubation, none of the NO-donors had a significant effect, neither on the expression of CFTR, nor on that of the ENaC subunits in the presence and absence of L-cysteine. It can be concluded that the effect of GSNO on Cl(-) efflux is, at least in part, due to its properties as an NO-donor, and the effect is likely to be mediated by CFTR, not by Ca(2+)-activated Cl(-) channels.

An acid-loading chloride transport pathway in the intraerythrocytic malaria parasite, Plasmodium falciparum

The intraerythrocytic malaria parasite exerts tight control over its ionic composition. In this study, a combination of fluorescent ion indicators and (36)Cl(-) flux measurements was used to investigate the transport of Cl(-) and the Cl(-)-dependent transport of "H(+)-equivalents" in mature (trophozoite stage) parasites, isolated from their host erythrocytes. Removal of extracellular Cl(-), resulting in an outward [Cl(-)] gradient, gave rise to a cytosolic alkalinization (i.e. a net efflux of H(+)-equivalents). This was reversed on restoration of extracellular Cl(-). The flux of H(+)-equivalents was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and, when measured in ATP-depleted parasites, showed a pronounced dependence on the pH of the parasite cytosol; the flux was low at cytosolic pH values < 7.2 but increased steeply with cytosolic pH at values > 7.2. (36)Cl(-) influx measurements revealed the presence of a Cl(-) uptake mechanism with characteristics similar to those of the Cl(-)-dependent H(+)-equivalent flux. The intracellular concentration of Cl(-) in the parasite was estimated to be approximately 48 mm in situ. The data are consistent with the intraerythrocytic parasite having in its plasma membrane a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive transporter that, under physiological conditions, imports Cl(-) together with H(+)-equivalents, resulting in an intracellular Cl(-) concentration well above that which would occur if Cl(-) ions were distributed passively in accordance with the parasite's large, inwardly negative membrane potential.

Direct evidence of chloride ion efflux in ischaemic and pharmacological preconditioning of cultured cardiomyocytes

AIMS

We have previously shown that reduction of ischaemic cell swelling via enhanced cell volume regulation is a key mechanism of ischaemic preconditioning (IPC) in cardiomyocytes. We have also shown that pharmacological blockade of Cl(-) channels abolishes cardioprotection achieved by IPC in Langendorff-perfused hearts and freshly isolated cardiomyocytes, thus suggesting that Cl(-) plays a key role in IPC cardioprotection. However, direct evidence of Cl(-) channel activation resulting in transsarcolemmal Cl(-) efflux by IPC had been lacking. To address this issue, 24 h cultured rabbit cardiomyocytes were loaded with 5 mM 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), a specific fluorescence indicator that is quenched by Cl(-) so that cellular efflux of Cl(-) results in an increase in SPQ fluorescence.

METHODS AND RESULTS

After stabilization for 10 min, cardiomyocytes were preconditioned either with 10 min simulated ischaemia/10 min simulated reperfusion or with 10 min treatment with 1 microM N(6)-2-(4-aminophenyl)ethyladenosine (APNEA). IPC and APNEA significantly (P < 0.001) reduced the intracellular Cl(-) concentration ([Cl(-)](i)) to 31.9 +/- 3.2 mM (mean +/- SEM) and 32.5 +/- 2.8 mM, respectively, from an initial [Cl(-)](i) (pooled stabilization 61.5 +/- 7.1 mM). [Cl(-)](i) did not change in control (non-preconditioned) cardiomyocytes (control 58.1 +/- 1.9 mM and control + vehicle 62.6 +/- 4.9 mM, P = 0.98 and 0.99 vs. pooled pre-treatment baseline, respectively). Inhibition of Cl(-) channels with 50 microM indanyloxyacetic acid 94 completely blocked preconditioning-induced Cl(-) efflux. Thus, a net Cl(-) efflux of 29.6 and 29.0 mM was triggered by IPC and APNEA.

CONCLUSION

These findings provide the first direct evidence of activation of sarcolemmal Cl(-) channels by ischaemic and pharmacological preconditioning in cardiomyocytes.

CFTR-mediated halide transport in phagosomes of human neutrophils

Chloride serves as a critical component of innate host defense against infection, providing the substrate for MPO-catalyzed production of HOCl in the phagosome of human neutrophils. Here, we used halide-specific fluorescent sensors covalently coupled to zymosan particles to investigate the kinetics of chloride and iodide transport in phagosomes of human neutrophils. Using the self-ratioable fluorescent probe specific for chloride anion, we measured chloride dynamics within phagosomes in response to extracellular chloride changes by quantitative fluorescence microscopy. Under the experimental conditions used, normal neutrophils showed rapid phagosomal chloride uptake with an initial influx rate of 0.31 +/- 0.04 mM/s (n=5). GlyH-101, a CFTR(inh), decreased the rate of uptake in a dose-dependent manner. Neutrophils isolated from CF patients showed a significantly slower rate of chloride uptake by phagosomes, having an initial influx rate of 0.043 +/- 0.012 mM/s (n=5). Interestingly, the steady-state level of chloride in CF phagosomes was approximately 26 mM, significantly lower than that of the control ( approximately 68 mM). As CFTR transports chloride as well as other halides, we conjugated an iodide-sensitive probe as an independent approach to confirm the results. The dynamics of iodide uptake by neutrophil phagosomes were monitored by flow cytometry. CFTR(inh)172 blocked 40-50% of the overall iodide uptake by phagosomes in normal neutrophils. In a parallel manner, the level of iodide uptake by CF phagosomes was only 20-30% of that of the control. Taken together, these results implicate CFTR in transporting halides into the phagosomal lumen.

Adenosine receptors in rat and human pancreatic ducts stimulate chloride transport

Previously, we have shown that pancreatic acini release adenosine triphosphate (ATP) and ATP-handling enzymes, and pancreatic ducts express various purinergic P2 receptors. The aim of the present study was to establish whether pancreatic ducts also express adenosine receptors and whether these could be involved in secretory processes, which involve cystic fibrosis transmembrane regulator (CFTR) Cl- channels or Ca2+-activated Cl- channels and H(+)/HCO(-)(3) transporters. Reverse transcriptase polymerase chain reaction analysis on rat pancreatic ducts and human duct cell adenocarcinoma lines showed that they express A1, A2A, A2B, and A3 receptors. Real-time PCR revealed relatively low messenger RNA levels of adenosine receptors compared to beta-actin; the rank order for the receptors was A2A>A2B>or=A3>>A1 for rat pancreas and A2B>A2A>>A3>or=A1 for duct cell lines. Whole-cell patch-clamp recordings on rat pancreatic ducts showed that, in about half of the recordings, adenosine depolarized the membrane voltage, and this was because of the opening of Cl- channels. Using a Cl--sensitive fluorophore and single-cell imaging on duct cell lines, it was found that 58% of PANC-1 cells responded to adenosine, whereas only 9% of CFPAC-1 cells responded. Adenosine elicited Ca2+ signals only in a few rat and human duct cells, which did not seem to correlate with Cl- signals. A2A receptors were localized in the luminal membranes of rat pancreatic ducts, plasma membrane of many PANC-1 cells, but only a few CFPAC-1 cells. Taken together, our data indicate that A2A receptors open Cl- channels in pancreatic ducts cells with functional CFTR. We propose that adenosine can stimulate pancreatic secretion and, thereby, is an active player in the acini-to-duct signaling.

Optical imaging of Ca2+-evoked fluid secretion by murine nasal submucosal gland serous acinar cells

Airway submucosal glands are sites of high expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and contribute to fluid homeostasis in the lung. However, the molecular mechanisms of gland ion and fluid transport are poorly defined. Here, submucosal gland serous acinar cells were isolated from murine airway, identified by immunofluorescence and gene expression profiling, and used in physiological studies. Stimulation of isolated acinar cells with carbachol (CCh), histamine or ATP was associated with marked decreases in cell volume (20 +/- 2% within 62 +/- 5 s) that were tightly correlated with increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) as revealed by simultaneous DIC and fluorescent indicator dye microscopy. Simultaneous imaging of cell volume and the Cl(-)-sensitive fluorophore SPQ indicated that the 20% shrinkage was associated with a fall of [Cl(-)](i) from 65 mm to 28 mm, reflecting loss of 67% of cell Cl(-) content, accompanied by parallel efflux of K(+). Upon agonist removal, [Ca(2+)](i) relaxed and the cells swelled back to resting volume via a bumetanide-sensitive Cl(-) influx pathway, likely to be NKCC1. Accordingly, agonist-induced serous acinar cell shrinkage and swelling are caused by activation of solute efflux and influx pathways, respectively, and cell volume reflects the secretory state of these cells. In contrast, elevation of cAMP failed to elicit detectible volume responses, or enhance those induced by submaximal [CCh], because the magnitude of the changes were likely to be below the threshold of detection using optical imaging. Finally, when stimulated with cholinergic or cAMP agonists, cells from mice that lacked CFTR, as well as wild-type cells treated with a CFTR inhibitor, exhibited identical rates and magnitudes of shrinkage and Cl(-) efflux compared with control cells. These results provide insights into the molecular mechanisms of salt and water secretion by lung submucosal glands, and they suggest that while murine submucosal gland fluid secretion in response to cholinergic stimulation can originate from CFTR-expressing serous acinar cells, it is not dependent upon CFTR function.

Halide fluxes in epithelial cells measured with an automated cell plate reader

A method is introduced to measure chloride permeability in cultured epithelial cells using 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-N-ethylquinolinium iodide quinolinium (MEQ) as fluorescent chloride-sensitive probes. The method involves growing cells in multiwell plates, incubating cells with SPQ or MEQ, and then exchanging intracellular or extracellular halide ions with nitrate. The resulting time course of SPQ or MEQ fluorescence is followed by repetitive readings with a multiwell fluorescence plate reader. Exchange times are extracted by fitting the time course with a single exponential function of time. The method was validated by measuring the effect of chloride channel activators and blockers in A6 and MDCK cells. The baseline iodide/nitrate exchange time was 200-300 s. Isoproterenol (a modulator of cAMP-activated chloride channels) increased the exchange rate by a factor of 1.4+/-0.1; A23187 (a modulator of calcium-activated chloride channels) increased the rate by 3.4+/-0.4; bradykinin (also a modulator of calcium-activated chloride channels) increased the rate by 2.0+/-0.4; forskolin (a direct stimulator of adenylate cyclase) increased the rate by 2.7+/-0.3. Diphenylamine-2-carboxylate (a chloride channel blocker) decreased the rate by 0.12+/-0.03. These results indicate that our method is a valid indicator of halide-nitrate exchange in cultured epithelial cells.

Activation of CFTR by genistein in human airway epithelial cell lines

Cystic fibrosis (CF) is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel expressed in epithelial cells. The effects of genistein and 4-phenylbutyrate (PBA) on CFTR were studied in three human airway epithelial cell lines expressing wild-type or DeltaF508 CFTR: Calu-3, CFSMEo-, and CFBE41o- cells. The cells were loaded with the fluorescent dye N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) and chloride efflux was studied. Forskolin and 3-isobutyl-1-methylxanthine (IBMX) induced chloride efflux in Calu-3 cells but not in CF lines. Genistein (2.5-50 microM) alone was able to induce chloride efflux in all cell lines. Genistein did not enhance the effect of forskolin and IBMX. PBA had little or no effect on genistein-induced chloride efflux. The effect of genistein seen at low concentrations makes genistein interesting for possible pharmacological treatment of CF, since it is known that similar concentrations can be obtained in plasma by a soy-rich diet.

Determination of chloride efflux by X-ray microanalysis versus MQAE-fluorescence

The importance of chloride channels for the cell is demonstrated by a number of serious human diseases that are due to mutations in chloride channels. The most well-known of these diseases is cystic fibrosis. Investigations into the mechanisms of the disease and possible treatments require the study of chloride fluxes at the level of individual cells. The present study compares two methods for studies of chloride transport: X-ray microanalysis and MQAE fluorescence with image analysis. As an experimental system, the cAMP-activated chloride channel in cultured respiratory epithelial cells was chosen. Both methods showed that stimulation with the cAMP-elevating agents forskolin and IBMX decreased the chloride content of the cells by about 20-27%. Inducing a driving force for chloride by replacing extracellular chloride by nitrate resulted in a chloride efflux that was significantly increased in the presence of forskolin and IBMX. This study shows that X-ray microanalysis and MQAE fluorescence are adequate and comparable methods for measuring cAMP-dependent chloride transport in individual cells.

S-Nitrosoglutathione induces functional DeltaF508-CFTR in airway epithelial cells

S-Nitrosoglutathione (GSNO) is an endogenous bronchodilator levels of which are reduced in the airways of cystic fibrosis (CF) patients. GSNO has recently been shown to increase maturation of CFTR in CF cell lines at physiological concentrations. The ability of S-nitrosoglutathione to direct the DeltaF508-CFTR to the plasma membrane and restore the function of the cAMP-dependent chloride transport in cultured human airway epithelial cells has been studied. Immunocytochemistry showed a time- and dose-dependent increase of apically located CFTR after GSNO treatment. Chloride transport studies with the fluorescent dye N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) showed that GSNO was able to induce a fourfold increase of cAMP-dependent chloride transport. Our data and the fact that endogenous GSNO levels are lower in the airways of CF patients make GSNO an interesting candidate for pharmacological treatment of cystic fibrosis.

Mechanisms of chloride transport in thymic lymphocytes

This study examined mechanisms of Cl- transport in rat lymphocytes under a variety of conditions. Basal intracellular Cl- concentration ([Cl-]i) was not different between cells assayed in the presence of HCO3- or its absence (HEPES). Removal of external Cl- resulted in a fall in [Cl-]i and a rapid rise in intracellular pH (pH(i)). Both Cl- efflux and the rise in pH(i) were blocked by DIDS or removal of external Na+ but were unaffected by furosemide. The mechanisms governing Cl- influx were assessed in cells that had been Cl- depleted for 1 h. Reexposure to Cl- resulted in a rapid rise in [Cl-]i that was partially inhibited by pretreatment with DIDS (57%) and partially inhibited by pretreatment with furosemide (45%). Pretreatment with both compounds together completely blocked Cl- influx. Cl- depletion caused a marked increase in pH(i) that rapidly declined toward normal when the cells were reexposed to Cl-. Preincubation with DIDS completely blocked this decrease in pH(i). In contrast, neither removal of Na+ nor preincubation with furosemide affected the decline in pH(i) when the cells were reexposed to Cl-. We conclude that, in thymic lymphocytes, Cl-/HCO3- (or Cl-/base exchange) regulates both Cl- influx and efflux. Cl- efflux is totally inhibited by DIDS and is mediated by a Na+-dependent Cl-/HCO3- exchanger. Cl- influx is partially DIDS sensitive and partially furosemide sensitive and is mediated by both a Na+-independent Cl-/HCO3- exchanger and by a Na+-K+-2Cl- cotransporter.

NSP4 elicits age-dependent diarrhea and Ca(2+)mediated I(-) influx into intestinal crypts of CF mice

Homologous disruption of the murine gene encoding the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) leads to the loss of cAMP-mediated ion transport. Mice carrying this gene defect exhibit meconium ileus at birth and gastrointestinal plugging during the neonatal period, both contributing to high rates of mortality. We investigated whether infectious mammalian rotavirus, the recently characterized rotaviral enterotoxin protein NSP4, or its active NSP4(114-135) peptide, can overcome these gastrointestinal complications in CF (CFTR(m3Bay) null mutation) mice. All three agents elicited diarrhea when administered to wild-type (CFTR(+/+)), heterozygous (CFTR(+/-)), or homozygous (CFTR(-/-)) 7- to 14-day-old mouse pups but were ineffective when given to older mice. The diarrheal response was accompanied by non-age-dependent intracellular Ca(2+) mobilization within both small and large intestinal crypt epithelia. Significantly, NSP4 elicited cellular I(-) influx into intestinal epithelial cells from all three genotypes, whereas both carbachol and the cAMP-mobilizing agonist forskolin failed to evoke influx in the CFTR(-/-) background. This unique plasma membrane halide permeability pathway was age dependent, being observed only in mouse pup crypts, and was abolished by either the removal of bath Ca(2+) or the transport inhibitor DIDS. These findings indicate that NSP4 or its active peptide may induce diarrhea in neonatal mice through the activation of an age- and Ca(2+)-dependent plasma membrane anion permeability distinct from CFTR. Furthermore, these results highlight the potential for developing synthetic analogs of NSP4(114-135) to counteract chronic constipation/obstructive bowel syndrome in CF patients.

Na+, K+, and Cl- transport in resting pancreatic acinar cells

To understand the role of Na+, K+, and Cl- transporters in fluid and electrolyte secretion by pancreatic acinar cells, we studied the relationship between them in resting and stimulated cells. Measurements of [Cl-]i in resting cells showed that in HCO3(-)-buffered medium [Cl-]i and Cl- fluxes are dominated by the Cl-/HCO3- exchanger. In the absence of HCO3-, [Cl-]i is regulated by NaCl and NaK2Cl cotransport systems. Measurements of [Na+]i showed that the Na(+)-coupled Cl- transporters contributed to the regulation of [Na+]i, but the major Na+ influx pathway in resting pancreatic acinar cells is the Na+/H+ exchanger. 86Rb influx measurements revealed that > 95% of K+ influx is mediated by the Na+ pump and the NaK2Cl cotransporter. In resting cells, the two transporters appear to be coupled through [K+]i in that inhibition of either transporter had small effect on 86Rb uptake, but inhibition of both transporters largely prevented 86Rb uptake. Another form of coupling occurs between the Na+ influx transporters and the Na+ pump. Thus, inhibition of NaK2Cl cotransport increased Na+ influx by the Na+/H+ exchanger to fuel the Na+ pump. Similarly, inhibition of Na+/H+ exchange increased the activity of the NaK2Cl cotransporter. The combined measurements of [Na+]i and 86Rb influx indicate that the Na+/H+ exchanger contributes twice more than the NaK2Cl cotransporter and three times more than the NaCl cotransporter and a tetraethylammonium-sensitive channel to Na+ influx in resting cells. These findings were used to develop a model for the relationship between the transporters in resting pancreatic acinar cells.

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl- of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl- transport rates (delta F/Fo s-1 (x10(-3))), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and Fo the maximal fluorescence (x10(-3)) of both membranes by following the intracellular Cl- activities (ai Cl-, measured with MQAE) after extracellular Cl- substitution (2) the blm 86Rb and 36Cl uptakes and (3) the cellular potential and Cl- current using the whole-cell patch-clamp technique to differentiate between the different Cl- transport mechanisms. The permeability of the blm to Cl- was found to be much greater than that of the apical membranes under resting conditions: aiCl- changes were 5.3 +/- 0.7 mM and 25.5 +/- 1.05 mM (n = 79) when Cl- was substituted by NO3(-) in the media bathing apical and basolateral membranes. The Cl- transport rate of the blm was blocked by bumetanide (100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 microM) but not by N-phenylanthranilic acid (DPC, 100 microM). 86Rb and 36Cl uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl- pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Appli-cation of a hypoosmotic medium to the serosal side of the cell increased delta F/Fo s-1 (x10(-3)) after extracellular Cl- substitution (1.03 +/- 0.10 and 2.45 +/- 0.17 arbitrary fluorescent units s-1 for isoosmotic and hypoosmotic conditions respectively, n = 11); this delta F/Fo s-1 (x10(-3)) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca2+ depletion had no effect on delta F/Fo s-1 (x10(-3)) under isoosmotic conditions, but blocked the delta F/Fo s-1 (x10(-3)) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was -37.2 +/- 4.0 mV but reached a maximal and transient depolarization of -25.1 +/- 3.7 mV (n = 9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of -85 mV (close to the Nernst equilibrium potential for K+) was blocked by NPPB and transiently increased by hypoosmotic shock (&ap;50% maximum increase). This study demonstrates that the major component of Cl- transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl- channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl- absorption and cell volume regulation. The increase in the blm Cl- conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the 86Rb efflux involved in cell volume regulation.

Regulated Cl transport, K and Cl permeability, and exocytosis in T84 cells

We measured stimulant-induced changes of exocytosis that are associated with increases in Cl secretion (i.e., short circuit current, ISC, in microA/cm2) and apical (ap) Cl permeability (PCl) and basolateral (bl) K permeability (PK) (both in cm/s) in T84 monolayers. PCl and PK were measured by permeabilizing the bl or ap membrane with nystatin. PCl was also measured with a fluorescent dye 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). A noninvasive and sensitive method (release of 35SO4-labeled glycosaminoglycan [GAG], a fluid-phase marker of Golgi-derived vesicles) was used to measure exocytosis at both ap and bl membranes. At rest, ISC = 3.6, PK = 0.8 x 10(-6), PCl = 2.1 x 10(-6) with SPQ and 2.4 x 10(-6) electrically, and there was constitutive GAG secretion (i.e., exocytosis) to both ap and bl sides (bl > 2 x ap). Carbachol (C) increased: ISC (delta = 18.6), PK (6.5x), PCl (1.8-2.9x), and exocytosis to both ap (2.2-3.5x) and bl (2.0-3.0x) membranes. Forskolin (F) increased ISC (delta = 29), PCl (5.5-11x) and ap exocytosis (1.5-2x), but had no effect on PK or bl exocytosis. Synergistic effects on ISC occurred when C was added to F-treated cells but not vice versa, even though the characteristic effects of F+C on PCl, PK, and/or GAG secretion were identical to those exhibited when stimulants were added individually. Cl secretion results from coordinated activation of channels at ap and bl membranes, and exocytosis may play a role in these events.

Adrenaline-regulated Cl- transport in cultured single rat epididymal cells measured by an entrapped Cl-(-)sensitive fluorophore

1. Isolated cells from primary cultures of rat epididymal epithelial cells were employed for the study of adrenaline-stimulated Cl- transport using a Cl-(-)sensitive fluorophore 6-methoxy-N-(3-sulphopropyl) quinolinium (SPQ). SPQ was loaded into the cells by the hypotonic shock method. 2. The resting intracellular Cl- concentration, estimated in the presence of nigericin and tributyltin in high-K+ solution, was 62.3 +/- 0.2 mM. This value was not altered in the presence of 1 microM adrenaline. When extracellular Cl- was replaced by NO3-, an increase in fluorescence corresponding to a decrease in intracellular Cl- was observed. The initial outward Cl- movement was estimated to be 0.54 +/- 0.08 mM s-1. This value was increased by incubating the cells with adrenaline. The stimulatory effect of adrenaline was reduced by 1 mM DPC. 3. Addition of Cl- to cells previously depleted of Cl- caused an instantaneous decrease in fluorescence due to the entry of Cl-. The initial rate of Cl- entry was -0.62 +/- 0.13 mM s-1. Adrenaline increased the rate of entry to -2.13 +/- 0.08 mM s-1. The adrenaline-stimulated rate of entry was reduced by DPC or frusemide (0.5 mM) and was completely blocked in the presence of both agents. 4. In Na(+)-free solution, the adrenaline-stimulated rise of rate of Cl- entry was reduced in the presence of DPC. Frusemide had no effect on the entry rate. 5. The stimulatory effect of adrenaline were abolished by propranolol (5 microM) but not by phentolamine (5 microM). Conversely, isoprenaline (1 microM) and forskolin (1 microM) mimicked the effects of adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinin-induced chloride permeability changes in colony 29 epithelia estimated from 125I- efflux and MEQ fluorescence

1. The changes in apical Cl- permeability of Colony 29 human colonic epithelial monolayers were estimated from the rate constant of 125I- efflux from tissues loaded with the isotope. 2. Forskolin was used to increase intracellular concentrations of adenosine 3:5' cyclic-monophosphate (cyclic AMP), and A23187 to increase intracellular free Ca2+ (Cai). Both treatments increased the rate constant for 125I- efflux, indicating an increase in apical Cl- permeability. 3. Lysylbradykinin (LBK) also increased the rate constant for 125I- efflux, sometimes biphasically. Chelation of intracellular Ca2+ with BAPTA or prevention of prostaglandin formation with piroxicam, attenuated but did not eliminate the effect of LBK. It is concluded that LBK affects 125I- efflux through the agency of both cyclic AMP and Ca2+. 4. Ba2+ attenuated the effect of LBK and A23187 on 125I- efflux, but had no effect on the action of forskolin. It is concluded that Ca2+ has a major effect on K+ channels, the resulting hyperpolarization increasing the driving force for 125I- efflux. A secondary effect on Ca(2+)-sensitive Cl- channels is possible. By contrast, cyclic AMP exerts it major effect on apical Cl- channels. 5. Using a Cl- sensitive fluorescent dye, MEQ, the intracellular chloride concentration, Cli was estimated to be around 30 mM, which was increased to around 50 mM by forskolin, suggesting cyclic AMP could activate the Na-K-2Cl co-transporter. 6. MEQ fluorescence was used to estimate Cl- influx and efflux rates of epithelial cells. These were increased three fold by forskolin and dibutyryl cyclic AMP and two fold by LBK and histamine. 7. It is concluded that LBK increases electrogenic chloride secretion in Colony 29 epithelia through the generation of second messengers cyclic AMP and Ca2+, each of which may act on both apical and basolateral membranes.

Synthesis of cell-impermeable Cl-sensitive fluorescent indicators with improved sensitivity and optical properties

Quinolinium compounds have been used as Cl-sensitive fluorescent indicators in cells and cell-free membrane fractions. To improve Cl sensitivity and for conjugation via nucleophilic reaction, the compounds 6-methoxy-N-(n-aminoalkyl)quinolinium bromide hydrochloride (AAQ) with alkyl chain lengths (n) of 2 (AEQ), 3 (APQ), and 4 (ABQ) were synthesized. AAQ was water soluble, fluorescent, and quenched by Cl. The Stern-Volmer constants (KCl) for quenching of protonated AEQ, APQ and ABQ by Cl were 354, 322, and 272 M-1, respectively, higher than KCl for 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ; 118 M-1). To eliminate pH-dependent fluorescence, 6-methoxy-N-(3-trimethylammoniumpropyl)quinolinium dibromide (TMAPQ) was synthesized (KCl, 310 M-1). To red shift fluorescence excitation and emission spectra, 6-phenyl-N-(3-trimethylammoniumpropyl) quinolinium dibromide (phenyl-TMAPQ) (emission 475 nm) and N-(3-trimethylammoniumpropyl)phenanthridinium dibromide (TMAPP) (excitation 380 nm) were synthesized. AEQ and ABQ were conjugated with neutral dextran activated by cyanogen bromide to give indicator-to-dextran mole ratios of 5 to 20. KCl values at pH 7.4 were 132 (AEQ-dextran) and 237 M-1 (ABQ-dextran). To construct a single molecule with Cl-sensitive and insensitive moieties, the bichromophores 6-methoxy-N-(n-dansylsulfonamidoalkyl)quinolinium with alkyl chains of two and four were synthesized. The new Cl-sensitive indicators were used for measurement of intracellular Cl activity and for the labeling of endocytic vesicles in 3T3 fibroblasts and T84 cells. Our results indicate that N-substitution of quinoline with positively charged moieties gives increased Cl sensitivity, and extension of ring conjugation gives indicators with red-shifted fluorescence spectra.

6-methoxy-N-(3-sulfopropyl)quinolinium; dextran; fibroblast

Submitted on December 20, 1990

Accepted on June 10, 1991

Niflumic and flufenamic acids are potent inhibitors of chloride secretion in mammalian airway

Effects of niflumic acid (NFA) and flufenamic acid (FFA), the two nonsteroid anti-inflammatory agents recently reported to inhibit Cl- current in Xenopus oocytes, were examined in cultured monolayers of dog and cow trachea. Both agents showed potent inhibition to the short-circuit current (Isc), an index of magnitude of transepithelial Cl- secretion, with values of Ki of 0.02 (for NFA) and 0.06 (for FFA) mM, respectively. The sensitivity sequence of Isc to the Cl- channel inhibitors tested was NFA > FFA > diphenylamine-2-carboxylate (DPC) >> anthracene-9-carboxylate (A9C). Thus, NFA and FFA are so far the most potent commercially available Cl- channel inhibitors tested in Cl(-)-secreting epithelia. The sensitivity sequence of 36Cl uptake to the above Cl- channel inhibitors in Xenopus laevis oocytes was found to be identical to the cultures of trachea. This seems to imply that the membrane Cl- channels of Xenopus oocytes are functionally similar to that identified in mammalian Cl(-)-secreting epithelia.

Hormonal regulation of Cl transport in polar airway epithelia measured by a fluorescent indicator

Cl transport mechanisms in polarized cultures of canine tracheal epithelium were examined using an Ussing-type chamber with independent mucosal and serosal perfusion. Cl activity was monitored continuously from fluorescence of entrapped 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). When added to the serosal (but not mucosal) solution, isoproterenol increased Cl fluxes across the apical membrane Cl more than fourfold. Apical Cl transport was sensitive to diphenylamine-2-carboxylate (DPC) but not to furosemide, whereas basolateral membrane Cl transport was sensitive to furosemide but not to DPC. Based on a mathematical model of Cl transport, we developed a sensitive protocol to measure hormone-sensitive Cl transport. In Cl-loaded cells in which basolateral Cl transport was partially inhibited by furosemide, mucosal Cl removal caused no Cl efflux before but rapid efflux (0.25 mM/s) after addition of isoproterenol or chlorophenylthio-cAMP. In the presence of indomethacin to block prostaglandin production, elevation of intracellular Ca by bradykinin or 4-bromo-A23187 did not cause Cl efflux, nor did Ca buffering with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid affect stimulation by the cAMP pathway. Phorbol 12-myristate 13-acetate increased Cl efflux submaximally (0.09 mM/s) but did not affect maximal stimulation by cAMP agonists. Methoxamine did not alter apical or basolateral membrane Cl transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-permeable fluorescent indicator for cytosolic chloride

A major limitation of quinolinium-based fluorescent indicators for cytosolic Cl- has been the necessity of invasive cell loading because the positively charged ring nitrogen confers high polarity and membrane impermeability. A novel approach to mask the positive nitrogen was developed and evaluated for rapid, noninvasive indicator loading into living cells and effective intracellular trapping. The nonpolar and lipophilic compound 6-methoxy-N-ethyl-1,2-dihydroquinoline (diH-MEQ) was Cl- insensitive but was readily oxidized to the membrane-impermeable and Cl(-)-sensitive fluorescent indicator 6-methoxy-N-ethylquinolium chloride (MEQ), MEQ had 344-nm absorbance and 440-nm emission maxima, 0.70 quantum yield, and 4100 M-1 cm-1 molar extinction coefficient. In aqueous buffers, the fluorescence of MEQ was quenched by Cl- by a collisional mechanism with a Stern-Volmer constant (KCl) of 145 M-1. MEQ fluorescence was quenched by other anions (KBr = 275 M-1, KI = 360 M-1, KSCN = 300 M-1) but not by NO3-, SO4(2-), cations, and pH. Swiss 3T3 fibroblasts and colonic T84 cells were loaded with MEQ by incubation at 37 degrees C with 25-50 microM diH-MEQ for 5-10 min followed by diH-MEQ-free buffer for 15 min. MEQ stained cells brightly and uniformly and was nontoxic in studies of cell growth, cAMP and Ca2+ signaling, and electrophysiological properties. MEQ leaked out of cells by less than 10% in 60 min and was sensitive to cytosolic Cl- with KCl = 19 M-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of cAMP-activated chloride currents by expression of CFTR

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. In order to evaluate its function, CFTR was expressed in HeLa, Chinese hamster ovary (CHO), and NIH 3T3 fibroblast cells, and anion permeability was assessed with a fluorescence microscopic assay and the whole-cell patch-clamp technique. Adenosine 3',5'-monophosphate (cAMP) increased anion permeability and chloride currents in cells expressing CFTR, but not in cells expressing a mutant CFTR (delta F508) or in nontransfected cells. The simplest interpretation of these observations is that CFTR is itself a cAMP-activated chloride channel. The alternative interpretation, that CFTR directly or indirectly regulates chloride channels, requires that these cells have endogenous cryptic, chloride channels that are stimulated by cAMP only in the presence of CFTR.

Intracellular chloride in submucosal gland cells

The chloride ion concentration within isolated tracheal submucosal gland cells was studied micro-spectrofluorometrically using a fluorescent dye, 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ), that is quenched by Cl-. Cells from normal weanling swine and from a cystic fibrosis (CF) patient were used. Ion substitution experiments showed that cell fluorescence increased in both cell types when bath Cl- was replaced with the impermeant anion glucuronate. Following a Donnan-type ion substitution that kept the product of the bath K+ and Cl- concentrations constant, reducing bath chloride had little effect on fluorescence for normal cells, but caused a marked increase for CF cells. Thus, K+ and Cl- ions have approximately the same Nernst potential in control submucosal gland cells; in contrast, cells from a CF patient concentrated Cli, resulting in a Cl- Nernst potential that was more positive than the K+ Nernst potential. This finding is consistent with the hypothesis that CF submucosal gland cells have a decreased Cl- permeability.

Expression of cystic fibrosis transmembrane conductance regulator corrects defective chloride channel regulation in cystic fibrosis airway epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) was expressed in cultured cystic fibrosis airway epithelial cells and Cl- channel activation assessed in single cells using a fluorescence microscopic assay and the patch-clamp technique. Expression of CFTR, but not of a mutant form of CFTR (delta F508), corrected the Cl- channel defect. Correction of the phenotypic defect demonstrates a causal relationship between mutations in the CFTR gene and defective Cl- transport which is the hallmark of the disease.

Development and biological applications of chloride-sensitive fluorescent indicators

Chloride movement across cell plasma and internal membranes, is of central importance for regulation of cell volume and pH, vectorial salt movement in epithelia, and, probably, intracellular traffic. Quinolinium-based chloride-sensitive fluorescent indicators provide a new approach to study chloride transport mechanisms and regulation that is complementary to 36Cl tracer methods, intracellular microelectrodes, and patch clamp. Indicator fluorescence is quenched by chloride by a collisional mechanism with Stern-Volmer constants of up to 220 M-1. Fluorescence is quenched selectively by chloride in physiological systems and responds to changes in chloride concentration in under 1 ms. The indicators are nontoxic and can be loaded into living cells for continuous measurement of intracellular chloride concentration by single-cell fluorescence microscopy. In this review, the structure-activity relationships for chloride-sensitive fluorescent indicators are described. Methodology for measurement of chloride transport in isolated vesicle and liposome systems and in intact cells is evaluated critically by use of examples from epithelial cell physiology. Future directions for synthesis of tailored chloride-sensitive indicators and new applications of indicators for studies of transport regulation and intracellular ion gradients are proposed.

Model of ion transport regulation in chloride-secreting airway epithelial cells. Integrated description of electrical, chemical, and fluorescence measurements

An electrokinetic model was developed to calculate the time course of electrical parameters, ion fluxes, and intracellular ion activities for experiments performed in airway epithelial cells. Model variables included cell [Na], [K], [Cl], volume, and membrane potentials. The model contained apical membrane Cl, Na, and K conductances, basolateral membrane K conductance, Na/K/2 Cl and Na/Cl symport, and 3 Na/2 K ATPase, and a paracellular conductance. Transporter permeabilities and ion saturabilities were determined from reported ion flux data and membrane potentials in intact canine trachea. Without additional assumptions, the model predicted accurately the measured short-circuit current (Isc), cellular conductances, voltage-divider ratios, open-circuit potentials, and the time course of cell ion composition in ion substitution experiments. The model was used to examine quantitatively: (a) the effect of transport inhibitors on Isc and membrane potentials, (b) the dual role of apical Cl and basolateral K conductance in cell secretion, (c) whether the basolateral symporter requires K, and (d) the regulation of apical Cl conductance by cAMP and Ca-dependent signaling pathways. Model predictions gave improved understanding of the interrelations among transporting systems and in many cases gave surprising predictions that were not obvious without a detailed model. The model developed here has direct application to secretory or absorptive epithelial cells in the kidney thick ascending limb, cornea, sweat duct, and intestine in normal and pathophysiological states such as cystic fibrosis and cholera.